1. Field of the Invention
The present invention relates generally to position sensors, and more specifically to a system and method for positioning an object using a Hall-effect position encoder, and still more specifically to a system and method for calibrating the positioning system.
2. Related Art
Magnetic tape drives are commonplace in today's computer industry. These tape drives are used to store digital information onto magnetic tapes and to subsequently read the stored information. Two examples of a magnetic tape drive are the IBM 3480 tape drive available from International Business Machines in Armonk, N.Y. and the StorageTek 4480 tape drive available from Storage Technology Corporation in Louisville, Colo. Another example is the AJ-D350 digital video tape recorder available from Panasonic Broadcast System Company in Secaucus, N.J.
Magnetic tapes are typically available in two common formats: the cassette and the cartridge. The cassette tape is a two-reel mechanism that includes a supply reel and a take-up reel. Cassette tape drives thread the magnetic tape along a transport path, past one or more magnetic transducer heads, and then transport the tape such that it travels along the transport path. The tape is taken from the supply reel and wound onto the take-up reel. For tape drives capable of operating in a forward and reverse direction, the take-up reel and the supply reel exchange "functions" when the direction is reversed.
For cartridge tapes, the take-up reel is external to the tape cartridge and is typically provided internal to the tape drive. When a cartridge is inserted into a tape drive, it is threaded along the transport path. The threading is accomplished as follows: an extractor arm "grasps" a leader block at the free end of the tape. The extractor arm pulls the tape by the leader block along the transport path past one or more magnetic transducer heads, idler arms, capstans, and the like. At the end of the threading operation, the tape leader block is inserted in a key in the take-up reel. Once this is accomplished, the tape can be transported along the transport path and read/write operations can take place.
To allow the cartridge tape leader block to be inserted in the take-up reel, the take-up reel must be positioned, or indexed, so that a key for accepting the leader block is aligned properly. One conventional technique for indexing the take-up reel is a mechanical system that employs a solenoid and a latch mechanism to align the take-up reel. This technique requires that mechanical hardware be attached to the take-up reel motor. This technique is often unreliable as it relies heavily on mechanical techniques. For example, large solenoids are often used which tend to wear more quickly than electronic components.
Another conventional technique for positioning a reel uses a position encoder to sense and indicate rotation of the motor shaft. One such position encoder uses Hall-effect devices to sense the movement of magnetized areas on the motor shaft. The Hall-effect device provides a phase and quadrature signal indicating speed and displacement of the motor shaft. These signals have an AC component that varies about a DC offset level. The AC component varies as a function of position. For example, the AC component may be a sinusoid, where each period of the sine wave indicates a certain amount of shaft displacement. To position the reel, a controller determines the actual position based on position encoder signals and moves the motor to position the wheel to the desired position.
Because the Hall-effect device has a DC offset level, conventional systems calibrate the output of the Hall-effect device to remove this offset or to adjust it to a predetermined level. It appears that conventional systems perform the calibration continuously. As a result, an incorrect offset level can be determined. This occurs when the motor stops turning the take up reel. The motor may stop at a point where the output of the Hall-effect device may be above or below the DC offset level. If this occurs, the calibration technique corrects for this non-DC level and not for the actual DC offset level. This results in a mis-calibration of the system. This problem is not limited to position encoders using Hall-effect devices but may arise in other positioning systems that provide signals that vary about a DC level.
What is needed is a reliable technique for indexing the take-up reel such that leader block insertion can be accomplished. What is further needed is a solution for determining an offset voltage of a position encoder and Hall effect device and calibrating the system for that offset voltage.